Coded pulse generating circuit



Sept. 13, 1955 w ALVAREZ 2,717,960

CODED PULSE GENERATING CIRCUIT Filed Sept. 26, 1951 4 Sheets-Sheet 1 T"if I TRANSMITTER RECEIVER TIMING GENERATOR FIG.I 38

SWITCH 39 ROTATING MECHANISM ZS TRANSMITTER RECEIVER I W I PULSE SWEEP32 GENERATOR GENERATOR INVENTOR LUIS W. ALVAREZ AT TOR/V5 Y p 13, 1955L. w. ALVAREZ 2,717,960

CODED PULSE GENERATING CIRCUIT Filed Sept. 26, 1951 4 Sheets-Sheet 2RECEIVER F l G 3 INVENTOR LUIS W. ALVAREZ BY QKJ A 7'TOR/VE Y S pt 1955w. ALVAREZ ,960

CODED PULSE GENERATING CIRCUIT Filed Sept. 26, 1951 4 Sheets-Sheet 3INVENTOR LUIS W. ALVAREZ ATTORNEY Sept. 13, 1955 L. w. ALVAREZ 2,717,960

CODED PULSE GENERATING CIRCUIT Filed Sept. 26, 1951 4 Sheets-Sheet 4 I00ID] INVENTOI? LUIS W. ALVAREZ A TTORNEY United States atent connr) PULSEGENERATING CIRCUIT Luis W. Alvarez, Berkeley, Calif., assignor, by mesneassignments, to the United States of America as represented by theSecretary of the Navy Application September 26, 1951, Serial No. 248,3477 Claims. (Cl. 250-36) This invention relates to radio beacons andespecially to such as are normally inoperative but may be fired fromremote points by radio signals.

One of the objects of the invention is to provide a beacon which willfire only when it receives a signal of a predetermined character andthen will send out a single high-speed coded signal.

Another object of the invention is to provide a beacon which can befired from an airplane and which will then send out a high-speed codedsignal enabling the pilot of the plane to identify the beacon.

Another object of the invention is to provide a system of guiding anairplane in flight using the beacon referred to in the paragraphs above.Still another object of the invention is to provide a system of guidingan airplane from an automatically fired beacon in which the identity ofthe beacon will be instantaneously observable in the airplane.

Other objects of the inventionand objects relating particularly to thearrangement and interconnection of the various parts will be apparent asthe description of the invention proceeds.

One embodiment of the invention has been illustrated in the accompanyingdrawings in which:

Fig. 1 is a diagrammatic representation of a complete system, showingthe beacon and the airplane apparatus which fires it, together with themeans for indicating the identity of the beacon at the airplane;

Fig. 2 is a front view of the vacuum tube indicator of Fig. 1, showingthe manner in which the indication is received; and

Figs. 3, 4 and 5 are cooperating portions of a circuit for producing thecoded signal which is radiated by the beacon.

Referring now to Fig. l, the beacon, which may preferably be located ata landing field or on an aircraft carrier, is shown as comprising atransmitter 16 which is provided with a suitable antenna 11 forradiating electromagnetic oscillations which may preferably be in themicrowave region. A timing generator 12 is connected to the transmitterand is arranged to modulate the oscillations produced thereby in apredetermined manner, so that a coded signal may be radiated from theantenna. For purposes of illustration, the signal modulated upon thecarrier waves has been indicated at 13 as two long pulses with a shortpulse in between them, representing the letter K in the InternationalMorse Code, and these pulses are fast enough so that the total time ofthe coded signal will be in the order of a hundred microseconds. Acircuit for producing this and other code letter combinations will besubsequently described.

A receiver 14 is connected to the timing generator 12 and this receiveris also supplied with an antenna 15 by means of which it may pick upsignals broadcast from some remote point.

Thereceiver is arranged so that it will respond only to signals of apredetermined character, as for instance, a double pulse modulation, asindicated at 16, the carrier ice frequency being the frequency to whichthe receiver 14 is tuned. The response of the receiver is used tocontrol the transmitter 10, so that the transmitter 10 is only operatingimmediately after one of the double pulses 16 has been received by thereceiver, and then in a manner and for a period of time sufficient onlyto radiate the coded signal 13, after which the transmitter is shut off.Therefore, when a double pulse at the proper carrier wave frequency ispicked up by the receiver 14, it fires the transmitter 10, radiating asingle coded signal, as determined by the timing generator 12.

The apparatus in the airplane for firing the beacon and also for pickingup the coded signal and translating the code into a visual indication,so that the beacon may be identified and located, also comprises atransmitter 25 and a receiver 26 (Fig. 1). As shown in the drawing, asingle antenna 27 provided with a suitable reflecting system to make theantenna directive is used for both the transmitter 25 and the receiver26. The transmitter 25 may be connected to the antenna 27 through aswitching device 28 which also serves to connect the receiver 26 withthe antenna 27. The switching device may be of the discharge type whichacts to connect the antenna 27 to the transmitter only when thetransmitter is operating and to connect the receiver 2d to the antenna27 at all other times.

The transmitter has connected to it a pulse generator 32 (Fig. 1) whichcontinuously produces the double pulses 16, already referred to, at, forexample, the rate of 2,000 of the double pulses per second. Thearrangement is such that a high-frequency electromagnetic wave isradiated from the antenna 27, only during the period the double pulse 16and the transmitter is shut off during the interval between successivedouble pulses. But the receiver is connected to the antenna at this timeand hence is free to receive signals picked up by the antenna when thetransmitter is not operating. The output of the receiver is connected tothe control grid terminal 33 of a cathode ray tube 34 which is used asthe indicator.

The cathode ray tube 34 may be of a standard type, and the electron beamthereof may be controlled by electromagnetic or electrostaticdeiiection. Electromagnetic defiection is illustrated in the drawing.The usual circuits for providing the necessary voltages for the variousother electrodes of the tube are of course provided but have beenomitted in the drawing.

A sweep circuit generator 35 is provided for the tube 34 and isconnected to suitable magnetic coils arranged in a sleeve positionedupon the neck of the tube 34, the current applied to the coils being inthe form of a saw-tooth wave arranged to move the electron beam alongone of the diameters of the face of the tube. The sweep generator isalso controlled by the pulse generator, so that the sawtooth waveproduced by the sweep generator will start to rise as the second pulseproduced by the pulse generator terminates. Thus the electron beamstarts to sweep across the tube when the transmitter shuts ofi. afterradiating the double pulse signal. The sweep current, however, is biasedin such a manner that the electron beam will move only from the centerof the face 37 of the tube out toward the periphery thereof, and thesweep generator is also connected to the receiver, so as to swing thecontrol grid of the cathode ray tube negative when the electron beam hasreached the periphery of the tube to render invisible the return of thebeam.

The antenna 27 and associated reflecting system may be positioned sothat a relatively narrow beam is directed at a desired angle ofelevation. In this position the antenna 27 and associated reflectingsystem may be rotated about a vertical axis, indicated at 38, by meansof a suitable mechanism 39 provided for that purpose. By means of thisrotating mechanism the antenna 27 may be rotated the antenna 27.Suitable slip rings may be used to make the electrical connections topermit the rotation of the sleeve. The arrangement is such that thesleeve 36 will rotate through 360, the speed of rotation beingproportional to the rotation of the antenna 27, even though the antenna27 does not rotate through 360. This results in i the path of theelectron beam in the cathode ray tube changing its angle as the sleeve36 is rotated and in proportion to the rotation of the antenna 27 Theoperation of the complete system is as follows:

The transmitter 25 produces an ultrahigh-frequency oscilis lation whichis modulated by the pulse generator 32 to produce the double pulsesignal 16. A double pulse is then radiated from the antenna 27 as itrotates on its vertical axis. The switching mechanism 28 thensubstantially disconnects the transmitter from the antenna L andconnects the antenna to the receiver until the next double pulse isproduced by the pulse generator which will then operate the transmitterto radiate another double pulse signal. Successive double pulse signalsare thus radiated from the antenna 27 at a rate which will be adjustedto the desired range of the system.

The antenna 27 continues to rotate until the radiated beam therefrompoints in the direction of the beacon and the energy of the concentratedbeam reaches the antenna 15 of the beacon receiver 14. Since thereceiver 14 is designed to respond only to the particular double pulsetransmitted by the airplane antenna 27, it will immediately energize thetransmitter 10 which, under control of its timing generator 12, willcause the beacon antenna 11 to radiate one sequence of the coded signal13. When the beacon is within the designed range of the airplaneapparatus, the signal from the beacon will be intercepted by the antenna27 on the airplane shortly after the double pulse has been transmittedand during the interval of time when the airplane receiver 26 isconnected to the antenna 27. Thus, the airplane receiver 26 will receivethe coded signal 13 and the control grid of the cathode ray tube 34 willbe swung positively for successive periods of time corresponding to thetime periods of the coded signal. which has meanwhile been moving fromthe center of the tube toward the periphery, will be intensified atthese periods of time and will produce bright spots on the face of thecathode ray tube during these time intervals. These will appear at 42 asa dash-dot-dash for the particular coded signal represented at 13.

At the instant when the signal is received from the beacon, the antenna27 will be pointed substantially at the beacon, and, as the magneticdeflection coil sleeve 36 is synchronized with the antenna 27, thedash-dot-dash on the face of the cathode ray tube will appear on a linewhich has an angle with respect, for instance, to a vertical linethrough the center of the tube, corresponding to the azimuth angle ofthe antenna 27, or, in other Words, to the direction of the beacon fromthe airplane. Since the electron beam of the cathode ray tube startsfrom the center of the tube each time the double pulse of thetransmitter is transmitted and moves at a uniform speed towards theperiphery, the point on the tube at which the coded signal appears willhave a distance from the a center of the tube proportional to thedistance the beacon is from the airplane. This is true because the timerequired for the radio signal from the airplane to get to the beacon,plus the time for the signal from the beacon to return to the airplane,is proportional to that distance.

The electron beam of the cathode ray tube 34, M

The antenna 27 now continues to rotate, and when it moves away from thedirection of the beacon the beacon receiver 14 will no longer pick upthe double pulse signal and the beacon will cease radiating its codesignal. No signal from that beacon will therefore be received by thereceiver 26 in the airplane until the antenna 27 has rotated through itscomplete cycle and is back again pointing in the direction of thebeacon. The double pulse radiated from the airplane antenna 27 will thenbe received again by the beacon receiver 14, and the beacon transmitterIt} will be fired, thus radiating the coded signal 13. Again the signal13 is picked up by the airplane antenna 27 and is fed to the receiver 26the response of which operates the control grid of the cathode ray tubeand the dash-dot-dash is again produced on the face of the cathode raytube in the same position as before (assuming there has beensubstantially little change in the position of the airplane). However,as the airplane is turned towards the beacon, the coded signal will movearound on the tube until it is in a position corresponding to the headonposition, which, for example, may be at the top of the tube on thevertical line through the center.

It will be understood that while the radiated beam from the antenna 27is sweeping across the beacon receiver antenna 15, several of thesignals from the airplane may be intercepted, and hence the beacon willsend out several complete coded signals at a time. This group ofsignals, thus produced, will operate merely to increase thecircumferential dimension of the dash-dot-dash indication on the face ofthe tube.

As the airplane approaches the beacon, the signal will continue to comein, but, because it will take less time for the signal from the beaconto reach the airplane, the visible indication on the tube will movetowards the center. The pilot of the plane is thus informed of thedirection of the beacon and the distance it is from his plane, and canguide his plane towards the beacon and, where the beacon is situated ata landing field, can locate the field and making a landing.

Although I have shown a means to rotate the electromagnetic deflectionsleeve 36 so as to control the direction of the sweep of the beam acrossthe face of the tube, it will be understood that this might be doneelectrically if desired. Also, the sweep of the electron beam may becontrolled by electrostatic deflection.

The transmitter 10 at the beacon may be operated at the same frequencyas the transmitter 25 in the airplane, and the double pulse signal 16will be reflected back from the beacon and will be received at thereceiver 26 at about the same time that the signal is received from thebeacon transmitter 10. However, the reflected signal will be much weakerthan that received from the beacon, since the en ergy of the reflectedsignal varies inversely with the fourth power of the distance betweenthe airplane and the beacon, while the energy received from the beacontransmitter varies inversely as the square of this distance. Therefore,there will be no substantial interference between the signals on theface of the tube 31. However, if

desired, the beacon transmitter 10 may be arranged to broadcast at afrequency different from that of the transmitter 25, and the airplanereceiver 26 may be tuned to the frequency of the transmitter 10. In thiscase, there will be no possibility of interference between the signalbroadcasted from the beacon and the reflected signal which istransmitted from the antenna 27 and reflected from the beacon. Aseparate antenna system for the receiver 26 may be used in this case.

I may prefer to design the beacon transmitter 10 to broadcast on a widerange of frequencies, so that airplanes equipped with receivers tuned todifferent frequencies may pick up the beacon signal. Also the beaconreceiver might have a broad band so as to receive and respond to doublepulse signals on different carrier frequencies.

Where the beacon is being used to guide an airplane, it is importantthat the time interval between the receipt of a double pulse signal bythe receiver 14 and the firing of the transmitter be reduced to aminimum and be held constant. The distance the airplane is from thebeacon, or, in other words, its range, is determined by measuring thetime required for the double pulse to leave the airplane and the codedsignal to be received. Therefore any delay in the time between thereceipt of the signal from the airplane and the firing of the beaconwill appear as an error in the range determination unless that delay isknown and can be taken into account.

It is also desired to get the coded signal from the beacon to theairplane before the next pulse is transmitted from the airplane. Thedouble pulse sent out by the airplane may occupy a time duration ofthree or four microseconds, leaving about 496 or 497 microsecondsbetween the radiation of the pulses, during which time the receiver isready to receive the coded signal back from the beacon. This is assuminga repetition rate of 2000 of the double pulses per second. Since thecathode ray beam starts from the center of the tube at the terminationof a double pulse and moves out towards the circumference, arriving atthe edge before the beginning of another pulse, it will be seen thatthis time interval determines the range of the apparatus. If we allow100 microseconds for the time duration of the coded signal, this signalfrom the beacon would have to be received at the airplane 100microseconds before the cathode ray beam reaches the edge of the tube inorder for the entire code to be reproduced. If we have 496 microsecondsavailable and 100 are needed for code, we would then have a range of theapparatus corresponding to 396 microseconds. This range for the figuresgiven would be roughly about 39 miles. The dot-and-dash line 43 in Fig.2 indicates the range limit of the system, as far as the coded beacon isconcerned, although in actual practice this line would come nearer thecircumference of the tube. If the beacon is at the maximum range of theapparatus, then the visibly indicated coded signal will be received, asindicated at 42 between the line 43 and the circumference of the tube.If the beacon is nearer than the maximum range, this signal will appearsomewhere nearer to the center of the tube. Even if the beacon isfarther than the maximum range, its position may be observed, becausethe first dash, or a portion of it, may appear at the rim of the tube,although the identification may not be complete.

In operating the system of the invention a number of beacons may beprovided, each with a different code, so that they can be identifiedfrom various remote points. Thus, if another beacon having a code signaldot-dash-dot were located at another direction from the airplane andnearer to it, the coded signal might appear as indicated at 44 on Fig.2. The presence of the other beacon indication on the cathode ray tubesgives that much more information to the pilot so that he can determinehis course very easily and accurately. Nevertheless, the observation ofonly one beacon gives the pilot both direction and distance; notriangulation is necessary.

Because of the persistence of vision, the operator will be able to seethe coded traces as if all the components were simultaneously made.However, the image may be made to last even longer, if desired, by usinga tube having a screen made of material which will retain the image fora longer period of time.

It will be understood that the cathode ray tube is especially useful asan indicator for the system of the invention because of the very smalltime intervals involved. The timethe cathode ray beam has to travel fromthe center of the tube to the periphery is measured in microseconds andthis rate of deflection may be easily attained in a cathode ray tube.

A system of radio echo detection has been used in airplanes. With such asystem a succession of discrete pulses may be radiated from the planewhile the directional antenna is scanning a field in space. If theradiated waves strike an object within the scanned field, they will bereflected back from the object and received by the airplane. Theposition of the antenna when the reflected signal is received will thengive the direction of the object, and the time required for the pulse toreach the airplane after being reflected from the object will representthe distance the object is from the airplane. Such radio echo detectionapparatus, if installed on an airplane, utilizes the same apparatus asdescribed-in con-' nection with the airplane equipment of thisinvention, with the exception that it is not necessary to send out adouble pulse from the antenna 27. It therefore becomes a practicalmatter to use the radio echo detection equipment in the airplane to sendout the pulses necessary to fire the radio beacon and also to receivethe signals from the beacon and indicate the range and direction. Ifdesired, the pulse generator 32 may be provided with a switch 45 andsuitable circuit connections so that the pulse generator may be causedto produce the double pulse when the switch is thrown one way and thesingle pulse for the radio echo detection when the switch is thrown theother way. Of course when the single pulse is transmitted, the beaconswill not respond. The same switch may retune the receiver 26 to thefrequency of the transmitter 10, if that frequency is not the same asthat of the transmitter 25.

The time duration of the coded signal radiated by the beacon ispreferably made as short as possible, since the sweep of the electronbeam in the indicator tube must include the time interval of the code inaddition to the time interval required for the airplane signal to travelto the beacon and the coded signal to travel from the beacon to theairplane. I prefer to produce the code within an interval ofmicroseconds, although, of course, this interval may be increased, ifdesired, depending on the particular application. Probably, it wouldnever need to exceed one millisecond.

Various circuits may be used for producing a coded signal within thistime interval. I may prefer, however, to use the circuit shown in Figs.3, 4, and 5. Each of these figures shows a part of the circuit, theconnections between the parts being indicated by letters. The circuitcomprises several distinct portions, each having a difierent function.Tubes 46, 47 and 48 are arranged to produce a substantially squarenegative pulse, indicated at 49. The tube 46 may be a pentode tubearranged as an amplifier to receive on its grid a positive pulse,indicated at 50, from the receiver 14, and to feed a negative pulse tothe grid circuit of the tube 47. The tubes 47 and 48 are connected toform a modified multivibrator circuit which does not oscillate but has acumulative effect to produce maximum conductivity in the tube 48whenever the tube 47 is cut off by a negative pulse on its grid. Thetube 48 will then remain conducting, as will be understood, for a periodof time depending on the time constant of the circuit, so that thenegative square pulse 49, produced in the plate circuit of the tube 48,will have a predetermined time duration, in this instance, correspondingin time to the desired time duration of the coded signal 13 of Fig. 1.

The negative square wave pulse 49 is then fed to the grid of a tube 51,which, together with tubes 52 and 53, forms a saw-tooth wave generator.The tube 51 may be a simple triode, and is connected in series with thetube 52, which is shown as a pentode tube arranged to operate atconstant plate current, regardless of the plate voltage. Since the tubes51 and 52 are in series, no voltage will be delivered to the plate ofthe tube 52 except when the tube 51 is conducting. A condenser 54 isconnected between the plate of the tube 52 and the ground, and the plateof the tube 52, is also connected through a coupling condenser 55 andinductance 56 to the grid of the tube 53, this grid being biased in theusual manner by a resistance 57.

The tube 51 is normally conducting, and since the resistance of the tube52 is relatively high and the condenser 54 is in parallel with it, thecondenser 54 will be charged to its fullest capacity and will remaincharged as, long as the tube 51 is conducting. During this time the tube53 is also conducting, since the grid of that tube is at cathodepotential.

Now, when the negative square pulse 49 is applied to the grid of thetube 51, this grid is driven below cut-ofi and removes the voltagesource from the plate of the tube 52. The condenser 54, being fullycharged, begins to discharge through the constant current tube 52, andas the voltage of the condenser decreases, the voltage on the grid ofthe tube 53 will also decrease, thus causing an increase of voltage onthe plate of that tube. The constant decrease of grid voltage on thetube 53 produces in effect a negative saw-tooth wave, as indicated at58, while the increase in voltage on the plate of the tube 53 producesthe positive saw-tooth wave, indicated at 59.

In Fig. 4 is shown a circuit for producing the successive sharp pulses,each separated from the other by a predetermined time duration, forstarting and stopping the pulses which form the component parts of thecoded signal 13. In this circuit a group of pentode tubes 62, 63, 64,65, 66, and 67, are shown, these tubes all being biased below cut-off,but in diiferent amounts, as will be explained. The grid of the tube 62is connected through the condenser 68 to the receiver 14 shown in Fig.3. This is indicated by the letter A appearing on the diagram of inegative pulse, indicated at 69, on the grid of the tube 1;

70, this grid being connected to the plate of the tube 62 through thesmall condenser 71.

The grid circuits of the tubes 63, 64, 65, 66 and 67 are similar. Eachreceives the positive saw-tooth wave 59 from the point marked B on Fig.3, which is the out- .1

put of the tube 53. This point is connected to the point marked B in thegrid circuit of each of the tubes 63 to 67 inclusive in Fig. 4. Betweenthis point and the grid of each tube is a condenser 72 and a resistor73. Also in each grid circuit a resistor 74 is connected between thejuncture of the resistor 73 and the condenser 72 and the movable arm 75of the potentiometer 76. One end of the winding of this potentiometer isconnected to ground and the other end to a source of negative potential.Adjustment of the arm 75 on the potentiometer will vary the grid bias onthe associated tube, so that the grid bias of the tubes 63 to 67inclusive may be adjusted to any desired cut-off value.

When the positive saw-tooth potential 59 is impressed on the grids ofthe tubes 63 to 67 inclusive, the grid potential of each tube will risealong the saw-tooth wave until the cut-off point has been reached, atwhich time the tube will become conductive and the plate voltage of thetube will drop to form a negative pulse, similar to the pulse 69, whichis then transferred through a condenser 77 to the tube 70 or to asimilar tube 78, depending on the particular connections. As has alreadybeen stated, the tube 62 is connected to the grid circuit of the tube70, so are also the tubes 64 and 66. The plate circuits oi the tubes 63,65, and 67 are connected to the grid circuit of the tube 78 and deliverpulses 69' to that circuit.

The tube 62 produces its pulse 69 as soon as it re ceives the pulse 50from the receiver 14. The grid bias of the tube 63 is adjusted, however,so that it will not become conductive until a period of time has elapsedcorresponding to the time interval of the first component, the dash, inthe coded signal 13. The bias of the tube 64 is adjusted so that thistube will not become conductive until an additional period of time haselapsed corresponding to the time interval between the end of the firstcomponent of the coded signal 13, the dash, and the beginning of thesecond component, the dot. The bias of the tube 65 is then adjusted sothat this tube will not become conductive until an additional period oftime has elapsed corresponding to the time duration of the dot in thecoded signal 13. In like manner the grid of the tube 66 is biased sothat this tube will not become conductive until an additional time haselapsed corresponding to the space between the dot and the second dashin the coded signal 13. And similarly, the grid bias of the tube 67 isadjusted so that that tube will not become conductive until anadditional time has elapsed corresponding to the time duration of thelast dash of the signal 13.

In this manner the tubes 62 to 67 inclusive start to conduct in sequenceafter the receiver 14 transmits its pulse 50, the starting of each tubecorresponding to the beginning or end of one of the coded signalcomponents. Since the tubes 62, 64 and 66 are connected to the tube 70,this tube will produce a positive pulse 80, corresponding to thebeginnings of the component parts of the coded signal, while, since thetubes 63, 65 and 67 are connected to the tube 78, this tube will producea positive pulse 80, corresponding to the ends of the component parts ofthe coded signal.

The circuit for transforming this succession of pulses 80 and 80,corresponding to the beginnings and the ends of the component parts ofthe coded signal 13, is shown in Fig. 5. In this circuit twomulti-electrode tubes 82 and 83 are connected together to form aflip-flop circuit having two conditions of operation: either one tube iscompletely off and the other tube is full on, or the second tube iscompletely off and the first tube is full on. Besides the plate 84 andthe cathode 85 of the tube 82, there are shown five separate grids, 86,87, 88, 89 and 90. The grids 90 and 88 are control grids which arecompletely shielded from each other and from the other elements by meansof the screen grids 87 and 89. The grid 86, next to the anode, is asuppressor grid. Similarly, the tube 83, in addition to its anode 92 andcathode 93, has five grids 94, 95, 96, 97 and 98. Of these the grids 96and 98 are control grids which are shielded by the screen grids and 97,the grid 94 being the suppressor grid.

The cathodes 85 and 93 and suppressor grids 86 and 94 of the two tubes82 and 83 are connected together and to a source of positive potential,indicated at 99. The plates 84 and 92 of the two tubes are connectedrespectively through resistors 100 and 101 to a source of still greaterpositive potential indicated at 102. The first control grids 90 and 98are connected respectively through biasing resistors 103 and 104 to thecathodes 85 and 93. The second control grids 88 and 96 respectively ofthe tubes 82 and 93 are connected through resistors 105 and 106respectively to the ground. All of the screen grids, 87, 89, 95, and 97are connected to a source of positive potential indicated at 107. Theplate 84 of the tube 82 is directly connected through a resistor 108 tothe second control grid 96 of the tube 83, the resistor 108 beingshunted by a condenser 109. Similarly, the plate 92 of the tube 83 isconnected through a resistor 110 to the second control grid 88 of thetube 82, the resistor 110 being shunted by a condenser 111.

The control grid 90 of the tube 82 is supplied with the output of thetube 70 of Fig. 4 through a suitable coupling condenser 112, theconnection between the output circuit of Fig. 4 and the grid circuit ofthe tube 82 being indicated by the letter C on both figures. Similarly,the output of the tube 78 of Fig. 4 is connected to the control grid 98of the tube 83 of Fig. 5 through a suitable coupling condenser 113, thisconnection being indicated by the letter D in both Figs. 4 and 5.

In the operation of the flip-flop circuit of Fig. 5, assume a positivepulse is applied to the control grid 90 of the tube 82. This increasesthe current flowing in the tube 82 with the result that the potential onthe plate 84 drops. The lowered potential on the plate causes the secondcontrol grid 96 of the tube 83 to be correspondingly lowered inpotential with the result that the current in the tube 83 is reduced.Reduction cf the current in the tube 83 causes the plate 92 of this tubeto increase in potential, and since this plate is connected to thecontrol grid 88 of the tube 82, this grid swings positively acorresponding amount, causing the tube 82 still further to increase inconductivity. The efiect is thus cumulative, a positive potentialapplied to the control grid 90 of the tube 83 causing the tube 82immediately to reach its maximum conductivity while the tube 83 is shutoff. This condition will maintain until something happens to change it.

If now a positive potential is applied to the control grid 98 of thetube 83, this tube starts to conduct. An increased current in the tube83 will cause the plate 93 thereof to drop in potential, which causesthe control grid 88 of the tube 82 to swing negative a correspondingamount, with the result that the current flow in the tube 82 isdecreased. This decrease of current through the tube 82 will cause itsplate 84' to increase in potential, with the result that the controlgrid 96 connected to it will correspondingly increase thus increasingthe current through the tube 83. Thus the tube 83 will immediatelybecome conductive and the tube 82 will be shut off.

The output of this flip-lop circuit is taken from the plate 92 of thetube 83 and is delivered through a suit able coupling condenser 115 tothe grid of a triode tube 116. It will be seen that as the pulses fromthe circuit of Fig. 4 are alternately and successively applied to thegrids of the tubes 82 and 83, the circuit will flip-flop back and forthand positive pulses will appear on the grid of the tube 116 which aresubstantially square and correspond in duration to the timing of thepulses produced by the circuit of Fig. 4. As shown, the signal thusapplied to the grid of the tube 116 will be the coded signal 13, alreadydescribed.

The tube 116, together with the additional tubes, 118, 119, 120, and121, may be included in the transmitter 10. The triode amplifier tube116 feeds an inverted coded signal into the grid cicuit of the tube 118from which a positive coded signal is applied to the control grid of thecathode follower tube 120, in the cathode circuit of which is placed theresistor 122. The potential drop across a portion of this resistor 122is applied between the anode and control grid of the tube 121 which maybe an oscillator of the velocity modulation type or other type ofoscillator capable of producing high-power oscillation. Because of thecathode follower tube 120, a positive coded signal appears on the gridof the oscillator tube and causes this tube to operate during thepositive portions of the coded signal. The diode 119 is connectedbetween the grid of the tube 120 and ground and is biased so as to clipoii any unwanted portions of the pulses which may be found above theflat tops thereof.

The circuits of Figs. 3, 4 and 5 are subject to considerable variation.For instance, in the saw-tooth generator comprising tubes 51, 52 and 53of Fig. 3, the constant current tube 52 is used where it is desired toproduce a linear saw-tooth wave. However, because the grids of the tubes63 to 67 in Fig. 4 may have biased potential, adjusted as desired,-'itis not necessary to produce a linear saw-tooth. Therefore, the tube 52may be omitted entirely and a resistance substituted in place of it,shunted across the condenser 54.

In Fig. 4 the tubes 62 to 67 are pentode vacuum tubes, but gas-filledtubes might be used, if desired. Such tubes are under the control oftheir grids only when the tubes are not conducting, and, when oncestarted conducting by positive pulses applied to the grids, willcontinue to conduct as long as they have sufficient plate potential tomaintain the conduction through the gas. With such tubes additionalmeans to shut off the 10 plate voltage after the coded signal isproduced would have to be provided.

From the above explanation it will already be clear that when a pulse isdelivered by the output of the receiver, the tubes 46, 47 and 48cooperate to produce a negative square pulse which is then applied tothe saw-tooth generator including the tubes 51, 52 and 53. These tubescooperate to transform the negative square pulse into a positivesaw-tooth which is then impressed on the grid circuits of tubes 63 to67, inclusive, the pulse from the receiver being impressed on the gridcircuit of the tube 62. The arrangement of grid biases of these tubescauses them to operate successively and the suc cessive operation of thetubes causes sharp pulses to be applied alternately to thecontrol gridcircuits of the tubes 82 and 83 in Fig. 5. This produces the flip-flopaction causing the succession of positive pulses 13 to appear in theoutput of this flip-flop circuit, the duration of these pulses dependingupon the timing of the tubes in the circuit of Fig. 4. This coded signalis again aniplified by the tubes 118 and 120 and impressed upon thecontrol grid of the transmitting oscillator, so that the oscillationsradiated by the antenna 11 are modulated in accordance with the codedsignal. As soon as the transmitter has radiated the single codecombination, nothing more happens until another pulse appears in theoutput circuit of the receiver 14. Thus the transmitter will send outits single coded signal only when the receiver responds.

It will be understood from a consideration of Figs. 4 and 5 that byadjusting the grid biases of the tubes 62 to 67 various combinations ofa coded signal involving three components may be produced, instead ofthe dash dot-dash signal given by way of example. It will also beevident that by omitting two of these tubes, combinations of twocomponents in the signal may be produced, while by adding additionalpairs of tubes, the circuit will operate to form additional componentsin the coded signal.

Many modifications of the invention may be made without departing fromthe spirit thereof and I do not.

desire therefore to limit myself to what has been shown.

and described except as such limitations occur in the appended claims.

What I desire to claim and secure by Letters Patent is:

1. A circuit for producing a plurality of pulses each having apredetermined time duration separated at predetermined time intervalscomprising, in combination, a plurality of thermionic tubes, means tobias the grid circuits of said tubes so that said tubes will operate atpredetermined different grid voltages, means to produce a saw-toothvoltage wave, means to apply said saw-tooth voltage wave to the gridcircuits of said tubes so that said tubes will operate in apredetermined sequence of operation, a flip-flop circuit having twoconditions of operation, means to connect alternate tubes in thesequence of operation to said flip-flop circuit in such a manner as tocause the operation of said tubes to produce one condition of operationof said flip-flop circuit, and means to connect the remaining tubes tosaid flipfiop circuit so as to produce the other condition of operationof said circuit when said tubes are operated.

2. A circuit for producing a plurality of pulses each with apredetermined time duration and having predetermined time intervalsbetween said pulses comprising, in combination, means to produce asaw-tooth voltage wave, a plurality of thermionic tubes, at flip-flopcircuit capable of two conditions of operation, means to cause one ofsaid thermionic tubes to operate at an early point in the formation ofsaid saw-tooth wave, means to cause the operation of that tube toproduce one condition of operation in said flip-flop circuit, means tocause a second thermionic tube to operate after a predetermined timeinterval of said saw-tooth wave, means to cause the operation of saidsecond thermionic tube to produce the other condition of operation ofsaid flip-flop circuit, means to cause a third thermionic tube tooperate after a predetermined time interval of said saw-tooth wave,means to cause the operation of said third thermionic tube to produceagain the first condition of operation of said fiip-fiop circuit, meansto cause a fourth thermionic tube to operate after a predetermined timeinterval of said saw-tooth wave, and means to cause the operation ofsaid fourth thermionic tube to produce again said second condition ofoperation of said flip-flop circuit.

3. A radio transmitter adapted to produce pulses of oscillations, eachpulse having a predetermined time duration and with predetermined timeintervals between pulses comprising, in combination, an oscillatingcircuit for producing carrier frequency oscillations, a fiip-fiopcircuit capable of having a first and a second condition of operation,means to cause said first condition of operation of said flip-flopcircuit to energize said oscillating circuit, means to cause said secondcondition of operation of said flip-flop circuit to de-energize saidoscillating circuit, a first thermionic tube, means to operate saidthermionic tube, means to cause the operation of said thermionic tube toproduce said first condition of operation of said flipfiop circuit, asecond thermionic tube, means to operate said second thermionic tube apredetermined time interval after said first thermionic tube hasoperated, means to cause the operation of said second thermionic tube toproduce said second condition of operation of said flip-flop circuit, athird thermionic tube, means to operate said third thermionic tube apredetermined time interval after the operation of said secondthermionic tube, means to cause the operation of said third thermionictube to produce said first condition of operation of said flip-flopcircuit, a fourth thermionic tube, means to operate said fourththermionic tube a predetermined time interval after the operation ofsaid third thermionic tube. and means to cause the operation of saidfourth thermionic tube to produce said second condition of operation ofsaid flip-flop circuit.

4. A circuit for producing a coded signal in response to a signal pulsecomprising, in combination, means responsive to said signal pulse forproducing a saw-tooth voltage wave, a plurality of thermionic tubes,means for biasing the grid circuits of said tubes to cause said tubes tooperate at different grid voltages, means for applying said saw-toothvoltage wave to the grid circuits of said tubes to cause said tubes tooperate in a predetermined time sequence, a flip-flop circuit having twoconditions of operation, means connecting alternate tubes in thesequence of operation to said flip-flop circuit and arranged to produceone condition of operation of said flip-flop circuit. and meansconnecting the remaining tubes to said flip-flop circuit and arranged toproduce the other condition of operation of said flip-flop circuit.

5. A system for producing a pulse train comprising, in combination,first, second and third thermionic tubes, means for biasing said tubesbeyond cutofi with the negative bias on successive tubes being ofincreasingly greater magnitudes, means for applying a saw-tooth voltagewave input signal to the control grids of said tubes whereby said tubesare sequentially rendered conductive, a bi-stable multivibrator havingfirst and second control grids, means for applying the output of saidfirst and third tubes to said first control grid whereby a firstcondition of operation is established at said multivibrator in responseto the conduction of said first or third tube and whereby the beginningof an output pulse in the coded pulse train is determined, and means forapplying the output of said second tube to said second control gridwhereby a second condition of operation is established at saidmultivibrator in response to the conduction of said second tube andwhereby the end Of an output pulse in the coded pulse train isdetermined.

6. A system for producing a train of coded pulses comprising, a seriesof thermionic tubes, means for biasing said tubes beyond cutotf with thenegative bias on successsive tubes being of increasingly greatermagnitudes, means for applying a positive saw-tooth wave to the controlgrids of said tubes whereby said tubes are sequentially renderedconductive, a bi-stable multivibrator having first and second controlgrids, means for connecting the odd numbered tubes in said series to thefirst control grid of said multivibrator whereby a first condition ofequilibrium is established at said multivibrator in response to theconduction of an odd numbered tube and whereby the beginning of apredetermined pulse in the coded pulse train is determined, and meansfor connecting the even numbered tubes in said series to the secondcontrol grid of said multivibrator whereby a second condition ofequilibrium is established at saidmultivibrator in response to theconduction of an even numbered tube and whereby the end of apredetermined pulse in the pulse train is determined.

7. In a system for generating a coded pulse train made up of unequallength pulses having unequal pulse spacings, the combination of a seriesof thermionic tubes, means for applying increasingly higher blockingpotentials to the control grids of successive tubes in said serieswhereby said tubes are normally maintained nonconductive, means forapplying a positive saw-tooth wave to the control grids of said tubeswhereby said tubes are sequentially unblocked, a bi-stable multivibratorhaving first and second control grids, means for applying the output ofthe odd numbered tubes in said series to said first control grid forinitiating a coded output pulse and means for applying the output of theeven numbered tubes in said series to said second control grid forterminating an output pulse, the difference in blocking potential on thecontrol grid of an odd and the next higher even tube in said seriesbeing so selected with respect to the slope of said saw-tooth wave thatthe time interval between conductivity of these tubes corresponds to thelength of duration of said pulses in said pulse train and that the timeinterval between conductivity of an even and the next higher odd tube insaid series corresponds to the length of duration of said pulse spacingsin said pulse train.

References Cited in the file of this patent UNITED STATES PATENTS1,979,484 Mathes Nov. 6, 1934 2,312,357 Odquist et a1 Mar. 2, 19432,331,418 Nolde Oct. 12, 1943 2,421,018 De Rosa May 27, 1947 2,522,551Williams Sept. 19, 1950 2,606,287 McCoy Aug. 5, 1952 OTHER REFERENCES ILoeifel: A Circuit for Firing Thyratrons in Timed Sequence, Review ofScientific Instruments, vol. 12, February 1941, pages 102 and 103.

